What form should I give my grandmother that gives her daughters POA after she passes?

Agree - what your grandmother actually wants to do is make a will, naming her daughters as her executors. If you search online for "making a will in [your state]" you will find all the information you need to give her. It should be straightforward and inexpensive, but if she wants to do it at all it's worth getting it done properly. Don't get scammed and don't let her be overcharged - pick a local firm or two, call them, and find out how much they charge for their will-writing service beforehand. Other forum members may know what the typical fee is?

As cwillie says, a POA is the wrong document/authority to have after death. All POA authority ends at the time of death of the principal. Does granny have a will? The executor of the will is the person who would handle all legal matters after death - and even though a will names a specific executor this person must have this formalized by a judge in a court of law. Once approved by a judge a Letter of Testamentary will be given to them - which is what banks, real estate brokers etc will need to see before any business can be conducted. The executor also needs to get a specific identification number from the IRS - which makes them an employee of the estate - also required to conduct any business. It's all a pain in the rear end and is best accomplished with the assistance of an attorney.

Your grandmother is mistaken, POA grants someone the ability to make decisions and administer the finances of someone who is still living. The only way to appoint someone to administer an estate after death is to write a Will and name that person as Executor (the title may be different where you live).
Neither POA nor a Will should be do it yourself projects, I would advise you to shop around for a lawyer.